Closest
Sends whichever idle transporter has the shortest journey to the pickup.
The journey is measured from the zone each transporter holds, in the direction it faces. A transporter part way along a link therefore counts the rest of that link: of two transporters queued on a link, the one in front is nearer. At the same distance, the transporter with fewer zones still to enter is preferred, which is the one in front when the junction ahead has no length.
The default, and the obvious policy: it minimises the empty running that a fleet does. Note that distance here is distance along the guide path, not separation in space. On a one-way loop a transporter standing a few feet past the pickup point may have to go all the way round, so choosing by straight-line distance would routinely send the wrong one -- quietly, and with no symptom other than a fleet that performs worse than it should.
Ties go to the earlier transporter in the candidate list, which is declaration order, so the choice is the same on every run.